The present invention relates to a liquid polysulfide compound which is useful in rubber compositions for use in a pneumatic tire.
Polysulfide rubbers have long been known in the art and are known to have certain outstanding properties such as excellent resistance to a wide range of solvents, good resistance to atmospheric oxidation and weathering, adherence to metals and retention of their elastic properties over a relatively wide temperature range. They were initially produced as solid polymers. At a later date, methods were developed (as disclosed, for example, in U.S. Pat. No. 2,466,963) for splitting the solid polymers to form liquid polythiopolymercaptan polymers that are curable to form rubber-like materials having the desirable properties described above. The liquid polymers are particularly useful in a wide variety of applications because of the ease of handling a liquid material and of forming it to a desired configuration in which it can be cured to become a shaped elastomeric product.
As disclosed in U.S. Pat. No. 2,466,963, the polysulfide polymer molecules are characterized by the recurring unit (RSS), in which R represents the same or different divalent organic radicals that can vary widely in their specific structure but are typically alkylene or oxyhydrocarbon radicals interconnected by disulfide groups. While a wide variety of such polymers can be prepared, the polymers that are presently of commercial importance fall within a somewhat more limited group. Several of the commercially important liquid polymers are particularly described in articles by Fettes and Jorczak, published in Industrial and Engineering Chemistry, vol 42, page 2217 (1950) and vol 43, page 324 (1951). As pointed out in these articles, the commercially available liquid polysulfide polymers (e.g., the LP-2, LP-3, LP-12, LP-31, LP-32 and LP-33 polymers, available from Morton International, Chicago, Ill.) (xe2x80x9cLPxe2x80x9d is a registered trademark of Thiokol Corporation) are generally prepared from bis-beta-chloroethyl-formal and have essentially the following structure:
HS"Parenopenst"C2H4OCH2OC2H4SS"Parenclosest"xC2H4OCH2OC2H4SH
wherein x has an average value of from about 2 to about 59, preferably from about 5 to about 20. Morton International also markets ELP-3 and ELP-33 which are the epoxy-terminated derivatives of the LP-3 and the LP-33 polysulfides, respectfully.
The present invention relates to a pneumatic tire having a rubber component containing a liquid polysulfide compound.
There is disclosed a pneumatic tire having a rubber component comprised of
(a) 100 parts by weight of at least one elastomer containing olefinic unsaturation; and
(b) 1 to 20 phr of a liquid organic polysulfide polymer having a molecular weight of from 500 to 10,000.
The present invention relates to a pneumatic tire having a rubber component containing elastomers having olefinic unsaturation. The phrase xe2x80x9crubber or elastomer containing olefinic unsaturationxe2x80x9d is intended to include both natural rubber and its various raw and reclaim forms as well as various synthetic rubbers. In the description of this invention, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d may be used interchangeably, unless otherwise prescribed. The terms xe2x80x9crubber composition,xe2x80x9d xe2x80x9ccompounded rubberxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and such terms are well known to those having skill in the rubber mixing or rubber compounding art. Representative synthetic polymers are the homopolymerization products of butadiene and its homologues and derivatives, for example, methylbutadiene, dimethylbutadiene and pentadiene as well as copolymers such as those formed from butadiene or its homologues or derivatives with other unsaturated monomers. Among the latter are acetylenes, for example, vinyl acetylene; olefins, for example, isobutylene, which copolymerizes with isoprene to form butyl rubber; vinyl compounds, for example, acrylic acid, acrylonitrile (which polymerize with butadiene to form NBR), methacrylic acid and styrene, the latter compound polymerizing with butadiene to form SBR, as well as vinyl esters and various unsaturated aldehydes, ketones and ethers, e.g., acrolein, methyl isopropenyl ketone and vinylethyl ether. Specific examples of synthetic rubbers include neoprene (polychloroprene), polybutadiene (including cis-1,4-polybutadiene), polyisoprene (including cis-1,4-polyisoprene), butyl rubber, styrene/isoprene/butadiene rubber, copolymers of 1,3-butadiene or isoprene with monomers such as styrene, acrylonitrile and methyl methacrylate, as well as ethylene/propylene terpolymers, also known as ethylene/propylene/diene monomer (EPDM), and in particular, ethylene/propylene/dicyclopentadiene terpolymers. The preferred rubber or elastomers are natural rubber, polybutadiene and SBR.
In one aspect, the rubber is preferably of at least two of diene-based rubbers. For example, a combination of two or more rubbers is preferred such as cis 1,4-polyisoprene rubber (natural or synthetic, although natural is preferred), 3,4-polyisoprene rubber, styrene/isoprene/butadiene rubber, emulsion and solution polymerization derived styrene/butadiene rubbers, cis 1,4-polybutadiene rubbers and emulsion polymerization prepared butadiene/acrylonitrile copolymers.
In one aspect of this invention, an emulsion polymerization derived styrene/butadiene (E-SBR) might be used having a relatively conventional styrene content of about 10 to about 28 percent bound styrene or, for some applications, an E-SBR having a medium to relatively high bound styrene content, namely, a bound styrene content of about 30 to about 45 percent.
The relatively high styrene content of about 30 to about 45 for the E-SBR can be considered beneficial for a purpose of enhancing traction, or skid resistance, of the tire tread. The presence of the E-SBR itself is considered beneficial for a purpose of enhancing processability of the uncured elastomer composition mixture, especially in comparison to a utilization of a solution polymerization prepared SBR (S-SBR).
By emulsion polymerization prepared E-SBR, it is meant that styrene and 1,3-butadiene are copolymerized as an aqueous emulsion. Such are well known to those skilled in such art. The bound styrene content can vary, for example, from about 5 to about 50 percent. In one aspect, the E-SBR may also contain acrylonitrile to form a terpolymer rubber, as E-SBAR, in amounts, for example, of about 2 to about 30 weight percent bound acrylonitrile in the terpolymer.
Emulsion polymerization prepared styrene/butadiene/acrylonitrile copolymer rubbers containing about 2 to about 40 weight percent bound acrylonitrile in the copolymer are also contemplated as diene based rubbers for use in this invention.
The solution polymerization prepared SBR (S-SBR) typically has a bound styrene content in a range of about 5 to about 50, preferably about 9 to about 36, percent. The S-SBR can be conveniently prepared, for example, by organo lithium catalyzation in the presence of an organic hydrocarbon solvent.
A purpose of using S-SBR is for improved tire rolling resistance as a result of lower hysteresis when it is used in a tire tread composition.
The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial for a purpose of enhancing the tire""s traction when it is used in a tire tread composition. The 3,4-PI and use thereof is more fully described in U.S. Pat. No. 5,087,668 which is incorporated herein by reference. The Tg refers to the glass transition temperature which can conveniently be determined by a differential scanning calorimeter at a heating rate of 10xc2x0 C. per minute.
The cis 1,4-polybutadiene rubber (BR) is considered to be beneficial for a purpose of enhancing the tire tread""s wear, or treadwear. Such BR can be prepared, for example, by organic solution polymerization of 1,3-butadiene. The BR may be conveniently characterized, for example, by having at least a 90 percent cis 1,4-content.
The cis 1,4-polyisoprene and cis 1,4-polyisoprene natural rubber are well known to those having skill in the rubber art.
The term xe2x80x9cphrxe2x80x9d as used herein, and according to conventional practice, refers to xe2x80x9cparts by weight of a respective material per 100 parts by weight of rubber, or elastomer.xe2x80x9d
The pneumatic tire of the present invention is of conventional design having (a) a carcass reinforced with biased or radially-extending cords, two axially-spaced bead portions, two axially-spaced sidewall portions, one adjacent to each bead portion and a crown portion intermediate the sidewall portions, (b) a circumferentially extending belt structure radially outwardly of the carcass at the crown portion and (c) a tread section radially outwardly of the belt structure. The rubber component of the tire of the present invention which contains the liquid organic polysulfide polymer may be located in the carcass, part of the belt structure and/or tread. For example, as part of the carcass, the component may be the apex, wirecoat, ply coat, squeegee compounds, gum strips, chafer, reinforcing sidewall inserts or exposed sidewall. As part of the tread section, the component may be the tread base or tread cap. The compound may also be the innerliner.
The rubber composition for use in the rubber component of the tire of the present invention contains a curable liquid polysulfide polymer. These polysulfide polymers have a molecular weight of from 500 to 10,000. Preferably, the polysulfide polymers have a molecular weight of from 1,000 to 8,000. Preferred liquid polysulfide polymers may be characterized by the general formula
HS"Parenopenst"C2H4OCH2OC2H4SS"Parenclosest"xC2H4OCH2OC2H4SHxe2x80x83xe2x80x83I
where x ranges from 4 to 59. The preferred polysulfides are characterized by x ranging from 5 to 50. Commercially available liquid polysulfide polymers of Formula I which may be used in the present invention include LP-2, LP-3, LP-12, LP-31, LP-32, LP-33, LP-977 and LP-980. These polysulfide materials are manufactured by Toray Thiokol K.K. and Morton International Inc.
The various commercially available liquid polysulfide polymers may be characterized by their mercaptan content (%), average molecular weight pour point and average viscosity in poises at 40xc2x0 C. LP-3 has a mercaptan content of 5.9-7.7 percent, an average molecular weight of 1000 and an average viscosity in poises of 90 at 40xc2x0 C. LP-33 has a mercaptan content of 5.0-6.5 percent, an average molecular weight of 1000, a pour point at xe2x88x9223xc2x0 C. and an average viscosity in poises of 165 at 4xc2x0 C. LP-977 and LP-980 each have a mercaptan content of 2.5-3.5 percent, an average molecular weight of 2500, a pour point at 4xc2x0 C. and an average viscosity in poises of 770 at 4xc2x0 C. LP-2 has a mercaptan content of 1.7-2.2 percent, an average molecular weight of 4000, a pour point at 70xc2x0 C. and an average viscosity in poise of 3800 at 4xc2x0 C. LP-32 has a mercaptan content of 1.5-2 percent, an average molecular weight of 4000, a pour point at 7xc2x0 C. and an average viscosity in poise of 3800 at 4xc2x0 C. LP-12 has a mercaptan content of 1.5-2.0 percent, an average molecular weight of 4000, a pour point at 7xc2x0 C and an average viscosity in poise of 3800 at 4xc2x0 C. LP-31 has a mercaptan content of 1.0-1.5 percent, an average molecular weight of 8000, a pour point at 10xc2x0 C. and an average viscosity in poise of 7400 at 4xc2x0 C.
In addition to the above liquid polysulfides of Formula I, epoxy-terminated polysulfides may be used. For the purposes of this invention, an epoxy-terminated polysulfide is a polysulfide having an epoxy group linked to the polymer chain at each of the two or more residual sulfhydryl sites. The epoxidation of one or a mixture of liquid polysulfides described above with a stoichiometric amount of an epichlorohydrin, dichlorohydrin, 1,2-dichloro-3-hydroxy propane, the diglycidyl ether of glycerol or a di-epoxylated novoloc resin may be used to obtain the epoxy-terminated liquid polysulfide. The epoxidation of a liquid polysulfide in an excess of epichlorohydrin is described in U.S. Pat. No. 5,173,549, which is incorporated herein by reference. Morton International""s ELB-3 and ELP-33, which are the epoxy-terminated derivatives of the LP-3 and LP-33 polysulfides are believed to be of the formula 
where x ranges from 4 to 59. The basic structure for ELP-612, another epoxy-terminated polysulfide that is available from Morton International is 
where R9 is the polysulfide compound and R10 is the epoxy component. ELP-3 has an epoxy equivalent weight of 600-800 and an average molecular weight of 1250 and ELP-612 has an epoxy equivalent weight of from 300-330.
The liquid polysulfide polymers used in the present invention may be added to the rubber by any conventional technique such as on a mill or in a Banbury. The amount of the liquid polysulfide polymer compound may vary widely depending on the type of rubber and other compounds present in the vulcanizable composition. Generally, the amount of the polysulfide polymer compound is used in a range of from about 1 to about 20 phr with a range of 3 to about 10 phr being preferred.
The rubber composition may contain a filler (such as silica, aluminosilicates, zeolites and/or carbon black) to contribute the desired properties of the rubber component. Such fillers may be used in conventional amounts ranging from 10 to 250 phr. For example, when used, the silica filler may be added in amounts ranging from 10 to 250 phr. Preferably, the silica is present in an amount ranging from 15 to 100 phr.
The commonly employed siliceous pigments used in rubber compounding applications can be used as the silica in this invention, including pyrogenic and precipitated siliceous pigments (silica) and aluminosilicates, although precipitate silicas are preferred. The siliceous pigments preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
Such silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600, and more usually in a range of about 50 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1938).
The silica may also be typically characterized by having a dibutylphthalate (DBP) absorption value in a range of about 100 to about 400, and more usually about 150 to about 300.
Further, the silica, as well as the aforesaid alumina and aluminosilicate may be expected to have a CTAB surface area in a range of about 100 to about 220. The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849 for set up and evaluation. The CTAB surface area is a well known means for characterization of silica.
Mercury surface area/porosity is the specific surface area determined by Mercury porosimetry. For such technique, mercury is penetrated into the pores of the sample after a thermal treatment to remove volatiles. Set-up conditions may be suitably described as using a 100 mg sample; removing volatiles during 2 hours at 105xc2x0 C. and ambient atmospheric pressure; ambient to 2000 bars pressure measuring range. Such evaluation may be performed according to the method described in Winslow, Shapiro in ASTM bulletin, p.39 (1959) or according to DIN 66133. For such an evaluation, a CARLO-ERBA Porosimeter 2000 might be used.
The average mercury porosity specific surface area for the silica should be in a range of about 100 to 300 m2/g.
A suitable pore-size distribution for the silica, alumina and aluminosilicate according to such mercury porosity evaluation is considered herein to be five percent or less of its pores have a diameter of less than about 10 nm; 60 to 90 percent of its pores have a diameter of about 10 to about 100 nm; 10 to 30 percent of its pores have a diameter of about 100 to about 1000 nm; and 5 to 20 percent of its pores have a diameter of greater than about 1000 nm.
The silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.
Various commercially available silicas may be considered for use in this invention such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhodia, with, for example, designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2, VN3, BV3380GR, etc, and silicas available from Huber, for example Huber Sil 8745.
As can be appreciated by one skilled in the art, it may be desirable to add to the silica containing rubber compound a sulfur containing organosilicon compound. Examples of suitable sulfur containing organosilicon compounds are of the formula:
Zxe2x80x94Alkxe2x80x94Snxe2x80x94Alkxe2x80x94Zxe2x80x83xe2x80x83II
in which Z is selected from the group consisting of 
where
R1 is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl;
R2 is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms;
Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8.
Specific examples of sulfur containing organosilicon compounds of Formula II which may be used in accordance with the present invention include: 3,3xe2x80x2-bis(triethoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(triethoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(triethoxysilylpropyl)octasulfide, 3,3xe2x80x2-bis(trimethoxysilylpropyl)tetrasulfide, 2,2xe2x80x2-bis(triethoxysilylethyl)tetrasulfide, 3,3xe2x80x2-bis(trimethoxysilylpropyl)trisulfide, 3,3xe2x80x2-bis(triethoxysilylpropyl)trisulfide, 3,3xe2x80x2-bis(tributoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(trimethoxysilylpropyl)hexasulfide, 3,3xe2x80x2-bis(trimethoxysilylpropyl)octasulfide, 3,3xe2x80x2-bis(trioctoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(trihexoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(tri-2xe2x80x3-ethylhexoxysilylpropyl)trisulfide, 3,3xe2x80x2-bis(triisooctoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(tri-t-butoxysilylpropyl)disulfide, 2,2xe2x80x2-bis(methoxy diethoxy silyl ethyl)tetrasulfide, 2,2xe2x80x2-bis(tripropoxysilylethyl)pentasulfide, 3,3xe2x80x2-bis(tricyclonexoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(tricyclopentoxysilylpropyl)trisulfide, 2,2xe2x80x2-bis(tri-2xe2x80x3-methylcyclohexoxysilylethyl)tetrasulfide, bis(trimethoxysilylmethyl)tetrasulfide, 3-methoxy ethoxy propoxysilyl 3xe2x80x2-diethoxybutoxysilylpropyltetrasulfide, 2,2xe2x80x2-bis(dimethyl methoxysilylethyl)disulfide, 2,2xe2x80x2-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3xe2x80x2-bis(methyl butylethoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(di t-butylmethoxysilylpropyl)tetrasulfide, 2,2xe2x80x2-bis(phenyl methyl methoxysilylethyl)trisulfide, 3,3xe2x80x2-bis(diphenyl isopropoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(diphenyl cyclohexoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(dimethyl ethylmercaptosilylpropyl)tetrasulfide, 2,2xe2x80x2-bis(methyl dimethoxysilylethyl)trisulfide, 2,2xe2x80x2-bis(methyl ethoxypropoxysilylethyl)tetrasulfide, 3,3xe2x80x2-bis(diethyl methoxysilylpropyl)tetrasulfide, 3,3xe2x80x2-bis(ethyl di-sec. butoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(propyl diethoxysilylpropyl)disulfide, 3,3xe2x80x2-bis(butyl dimethoxysilylpropyl)trisulfide, 3,3xe2x80x2-bis(phenyl dimethoxysilylpropyl)tetrasulfide, 3-phenyl ethoxybutoxysilyl 3xe2x80x2-trimethoxysilylpropyl tetrasulfide, 4,4xe2x80x2-bis(trimethoxysilylbutyl)tetrasulfide, 6,6xe2x80x2-bis(triethoxysilylhexyl)tetrasulfide, 12,12xe2x80x2-bis(triisopropoxysilyl dodecyl)disulfide, 18,18xe2x80x2-bis(trimethoxysilyloctadecyl)tetrasulfide, 18,18xe2x80x2-bis(tripropoxysilyloctadecenyl)tetrasulfide, 4,4xe2x80x2-bis(trimethoxysilyl-buten-2-yl)tetrasulfide, 4,4xe2x80x2-bis(trimethoxysilylcyclohexylene)tetrasulfide, 5,5xe2x80x2-bis(dimethoxymethylsilylpentyl)trisulfide, 3,3xe2x80x2-bis(trimethoxysilyl-2-methylpropyl)tetrasulfide, 3,3xe2x80x2-bis(dimethoxyphenylsilyl-2-methylpropyl)disulfide.
The preferred sulfur containing organosilicon compounds of Formula II are the 3,3xe2x80x2-bis(trimethoxy or triethoxy silylpropyl)sulfides. The most preferred compounds are 3,3xe2x80x2-bis(triethoxysilylpropyl) tetrasulfide and 3,3xe2x80x2-bis(triethoxysilylpropyl) disulfide. Preferably Z is 
where R2 is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularly preferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3 carbon atoms being particularly preferred; and n is an integer of from 2 to 4.
The amount of the above sulfur containing organosilicon compound of Formula II in a rubber composition will vary depending on the level of silica that is used. Generally speaking, the amount of the compound of Formula II will range from 0 to 1.0 parts by weight per part by weight of the silica. Preferably, the amount will range from 0 to 0.4 parts by weight per part by weight of the silica.
The commonly employed and commercially available carbon blacks used in rubber compounding applications can be used in the compositions of the present invention. Representative examples of such carbon blacks include those known by the following ASTM designations, N110, N121, N134, N220, N231, N234, N242, N293, N299, S315, N326, N330, M332, N339, N343, N347, N351, N358, N375, N472, N539, N550, N582, N630, N642, N650, N660, N683, N754, N762, N765, N774, N787, N907, N908, N990 and N991. When carbon black is used, the amount may vary. Generally speaking, the amount of carbon black may vary from 0 to 250 phr. Preferably, the amount of carbon black will range from 0 to 150 phr. It is to be appreciated that a silica coupler may be used in conjunction with a carbon black (namely, pre-mixed with a carbon black prior to addition to the rubber composition) and such carbon black is to be included in the aforesaid amount of carbon black for the rubber composition formulation.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, sulfur donors, curing aids, such as activators and retarders and processing additives, such as oils, resins including tackifying resins and plasticizers, modified starches, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants and peptizing agents. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts. Typical amounts of reinforcing type carbon blacks(s), for this invention, if used, are herein set forth. Representative examples of sulfur donors include elemental sulfur (free sulfur), an amine disulfide, polymeric polysulfide and sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. The sulfur vulcanizing agent may be used in an amount ranging from 0.5 to 8 phr, with a range of from 0.5 to 6 phr being preferred. Typical amounts of tackifier or prereacted resins comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids comprise about 1 to about 50 phr. Such processing aids can include, for example, aromatic, napthenic and/or paraffinic processing oils. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, monophenols, bisphenols and thiobisphenols, polyphenols, hydroquinones derivatives, phosphites, thioesters, naphthylamines, diphenylamine derivatives, para-phenylenediamines, quinolines and others, such as, for example, those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to 5 phr. Representative examples of such antiozonants may be, for example, para-phenylenediamines such as diaryl-p-phenylenediamines, dialkyl-p-phenylenediamine and alkyl-aryl-p-phenylenediamines. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline and paraffinic waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
In one aspect of the present invention, the sulfur vulcanizable rubber composition is then sulfur-cured or vulcanized.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. The primary accelerator(s) may be used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 3.0, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts, such as from about 0.05 to about 3 phr, in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Peroxide curatives may also be present. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The rubber compositions of the present invention may contain a methylene donor and a methylene acceptor. The term xe2x80x9cmethylene donorxe2x80x9d is intended to mean a compound capable of reacting with a methylene acceptor (such as resorcinol or its equivalent containing a present hydroxyl group) and generate the resin in-situ. Examples of methylene donors which are suitable for use in the present invention include hexamethylenetetramine, hexaethoxymethylmelamine, hexamethoxymethylmelamine, lauryloxymethylpyridinium chloride, ethoxymethylpyridinium chloride, trioxan hexamethoxymethylmelamine, the hydroxy groups of which may be esterified or partly esterified, and polymers of formaldehyde such as paraformaldehyde. In addition, the methylene donors may be N-substituted oxymethylmelamines, of the general formula: 
wherein X is an alkyl having from 1 to 8 carbon atoms, R3, R4, R5, R6 and R7 are individually selected from the group consisting of hydrogen, an alkyl having from 1 to 8 carbon atoms and the group xe2x80x94CH2OX. Specific methylene donors include hexakis-(methoxymethyl)melamine, N,Nxe2x80x2,Nxe2x80x3-trimethyl/N,Nxe2x80x2,Nxe2x80x3-trimethylolmelamine, hexamethylolmelamine, N,Nxe2x80x2,Nxe2x80x3-dimethylolmelamine, N-methylolmelamine, N,Nxe2x80x2-dimethylolmelamine, N,Nxe2x80x2,Nxe2x80x3-tris(methoxymethyl)melamine and N,Nxe2x80x2Nxe2x80x3-tributyl-N,Nxe2x80x2,Nxe2x80x3-trimethylol-melamine. The N-methylol derivatives of melamine are prepared by known methods.
The amount of methylene donor and methylene acceptor that is present in the rubber stock may vary. Typically, the amount of methylene donor and methylene acceptor that each is present will range from about 0.1 phr to 10.0 phr. Preferably, the amount of methylene donor and methylene acceptor that each is present ranges from about 2.0 phr to 5.0 phr.
The weight ratio of methylene donor to the methylene acceptor may vary. Generally speaking, the weight ratio will range from about 1:10 to about 10:1. Preferably, the weight ratio ranges from about 1:3 to 3:1.
When the compound of the present invention is used as a wire coat or bead coat for use in a tire, an organo-cobalt compound may be present which serves as a wire adhesion promoter. When used, any of the organo-cobalt compounds known in the art to promote the adhesion of rubber to metal may be used. Thus, suitable organo-cobalt compounds which may be employed include cobalt salts of fatty acids such as stearic, palmitic, oleic, linoleic and the like; cobalt salts of aliphatic or alicyclic carboxylic acids having from 6 to 30 carbon atoms; cobalt chloride, cobalt naphthenate; cobalt carboxylate and an organo-cobalt-boron complex commercially available under the designation Manobond C from Wyrough and Loser, Inc, Trenton, N.J. Manobond C is believed to have the structure: 
in which R8 is an alkyl group having from 9 to 12 carbon atoms.
Amounts of organo-cobalt compound which may be employed depend upon the specific nature of the organo-cobalt compound selected, particularly the amount of cobalt metal present in the compound. Since the amount of cobalt metal varies considerably in organo-cobalt compounds which are suitable for use, it is most appropriate and convenient to base the amount of the organo-cobalt compound utilized on the amount of cobalt metal desired in the finished stock composition. Accordingly, it may in general be stated that the amount of organo-cobalt compound present in the stock composition should be sufficient to provide from about 0.01 percent to about 0.35 percent by weight of cobalt metal based upon total weight of the rubber stock composition with the preferred amounts being from about 0.03 percent to about 0.2 percent by weight of cobalt metal based on total weight of skim stock composition.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example the ingredients are typically mixed in at least two stages, namely at least one non-productive stage followed by a productive mix stage. The final curatives including sulfur vulcanizing agents are typically mixed in the final stage which is conventionally called the xe2x80x9cproductivexe2x80x9d mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber, silica, compound of Formula II and carbon black, if used, are mixed in one or more non-productive mix stages. The terms xe2x80x9cnon-productivexe2x80x9d and xe2x80x9cproductivexe2x80x9d mix stages are well known to those having skill in the rubber mixing art. The liquid polysulfide may be added at any stage of mixing but is preferably added in a nonproductive stage. The rubber composition containing the rubber and generally at least part of the silica should, as well as the sulfur-containing organosilicon compound of Formula II, if used, be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises a mechanical working in a mixer or extruder for a period of time suitable in order to produce a rubber temperature between 140xc2x0 C. and 190xc2x0 C. The appropriate duration of the thermomechanical working varies as a function of the operating conditions and the volume and nature of the components. For example, the thermomechanical working may be from 1 to 20 minutes.
The above tread rubber composition is used to prepare an assembly of a tire with a tread comprised of the said rubber composition. Such tire is then vulcanized.
Accordingly, the invention contemplates a vulcanized tire prepared by with the liquid organic polysulfide described herein.
Vulcanization of the pneumatic tire of the present invention is generally carried out at conventional temperatures ranging from about 100xc2x0 C. to 200xc2x0 C. Preferably, the vulcanization is conducted at temperatures ranging from about 110xc2x0 C. to 180xc2x0 C. Any of the usual vulcanization processes may be used such as heating in a press or mold, heating with superheated steam or hot air or in a salt bath.
The pneumatic tire of the present invention may be a passenger tire, aircraft tire, agricultural, earthmover, off-the-road, truck tire and the like. Preferably, the tire is a passenger or truck tire. The tire may also be a radial or bias, with a radial tire being preferred.